Silicone elastomers and pressure sensitive adhesives (PSAs) are known for their chemical inertness and resistance to weathering. Other characteristics include retention of elastomeric character at low temperature, resistance to thermal degradation and retention of good mechanical properties at elevated temperature, a low dielectric constant, and excellent pressure sensitive adhesion to low energy surfaces. Thus, these materials are well-suited to demanding industrial applications and find wide use in the electrical and aerospace industries.
Silicone elastomers have traditionally been prepared by compounding gums of high molecular weight polyorganosiloxanes, filler, processing aids, and peroxide curing agents. The resulting composition is then cured at elevated temperature, i.e., from about 150.degree. C. to about 250.degree. C., depending upon the peroxide utilized. Drawbacks of such high temperature vulcanizable elastomers include the difficulty of processing or milling the high molecular weight gum and silica, the high temperature requirement, and, sometimes, the need for high pressure, as well. Silicone PSAs have been prepared similarly but with MQ tackifying resin substituted for the filler. However, there are several major disadvantages associated with this method of preparation of silicone PSAs. First of all, the mixture must be applied from solvent to improve its processability. This necessitates drying ovens and pollution abatement equipment, and it also places limitations on coating thickness due to the difficulty of rapidly removing solvent without generating bubbles or imperfections in thick films. Secondly, curing at an elevated temperature precludes the use of many substrate materials which do not possess sufficient heat stability. Finally, the cure may variably continue for days or weeks after the thermal treatment, leading to increased crosslink densities. This is a particularly troublesome problem for silicone PSAs which, upon aging, show decreased peel adhesion and tack properties. Room temperature vulcanizable (RTV) elastomers have been developed but have, in general, required lengthy cure times in order to obtain complete cure or have exhibited inferior properties. Thus, there has been a recognized need in the art (see, e.g., U.S. Pat. No. 4,675,346) for solventless silicone compositions with good processability that cure rapidly and completely at moderate temperatures to elastomers or PSAs possessing good and stable properties.
Workers in the art have looked upon radiation curing as a means of overcoming the above-mentioned disadvantages and have functionalized silicone gums in various ways to allow for cure with actinic radiation at moderate temperature. This has been successfully applied to low molecular weight gums (i.e., gums which cure to provide a low molecular weight between crosslinks) as disclosed in U.S. Pat. Nos. 4,369,300 (Carter et al.), 4,563,539 (Gornowicz et al.), and 4,605,712 (Mueller et al.) and by Yu et al. in J. Appl. Polym. Sci. 30, 2115 (1985). However, the materials resulting from the curing of these low molecular weight gums have a high crosslink density and therefore do not possess good elastomeric properties. Similar problems are observed in U.S. Pat. No. 4,370,358 (Hayes et al.) which describes an ultraviolet (UV) light curable silicone PSA derived from epoxy functional silicone polymer (chosen from a range of molecular weights) in admixture with MQ silicone resin and a cationic photoinitiator. Although acceptable peel adhesion values are shown immediately after curing for the lower molecular weight PSAs, a large drop in peel values occurs upon aging at room temperature. This is indicative of the continued curing which is a typical problem with cationic systems. The low peel values ultimately obtained reflect a high crosslink density (low molecular weight between crosslinks) and less than optimum elastomeric character. U.S. Pat. No. 4,777,276 (Rasmussen et al.) also concerns low molecular weight materials, disclosing acrylamido- and methacrylamido-acyl oligomers which are the acrylamido-acyl and methacrylamido-acyl derivatives of amino-, hydroxyl-, and thiol-substituted polyoxyalkylene, polyalkyleneimine, polyester, polyolefin, polyacrylate, polyamide, polymerized fatty acids, and polysiloxane oligomers having at least one hydroxyl, thiol, or primary or secondary amino group and a molecular weight of about 200 to about 20,000.
It has long been known (Lewis, Rubb. Chem. Tech. 35, 1222 [1962]) that to obtain good elastomeric properties in a traditional peroxide-cured silicone rubber that there should be between about 200 and about 600 monomer units, i.e., a molecular weight of from about 15,000 to about 45,000, between crosslinks. Accordingly, efforts have been made to increase the molecular weight between the functional sites which lead to crosslinks in the radiation curable silicone systems. The problem has been that, as the molecular weight between reactive functional sites is increased, the concentration of reactive functionality is diluted, giving systems in which rapid and complete cure is difficult, if not impossible, to achieve. This is illustrated in U.S. Pat. No. 4,640,940 (Jacobine et al.) which gives examples which show that, as the molecular weight of a (meth)acrylate-terminated polydimethylsiloxane is increased from 1,700 to 5,000 to 12,000 to 28,000, the required cure time greatly increases and the degree of cure (measured as Durometer Shore A hardness) falls off. U.S. Pat. No. 4,675,346 (Lin et al.) discloses UV curable silicone compositions containing linear silicone resin (of at least about 150 siloxane units) having terminal acrylic groups, at least about 10% of a reinforcing fumed silica filler, and a photoinitiator. This reference states that, as molecular weight increases, the decreasing acrylic function density increases the difficulty of UV cure until the composition becomes uncurable with silicones above about 50,000 molecular weight. These systems are further described in U.S. Pat. Nos. 4,575,545 (Nakos et al.) and 4,575,546 (Klemarczyk et al.) as being in general difficult, if at all possible, to cure with chemical free radical generators at ambient temperatures, due to the low acrylic functionality density of the resins. Materials "having as a central feature a characteristic of having at least two terminal acrylate unsaturations and an organosilicone containing backbone" are also described in European Patent Publication No. 170219, published Feb. 5, 1986 (Dentsply).
One approach to improving curability has been to increase the density of reactive functionality by placing multiple reactive groups on a given siloxane unit. Silicone compositions reflecting this approach are disclosed in U.S. Pat. Nos. 4,503,208 (Lin et al.) and 4,640,940 (Jacobine et al.), as well as in U.S. Pat. Nos. 4,293,397 (Sato et al.), 4,364,809 (Sato et al.), 4,591,608 (Okinoshima), and 4,603,086 (Fujii et al.). The use of multiple groups improves cure rate to some extent over the use of "monofunctional" materials, yet the reported curing rates are still longer than desired for an industrially viable process.
U.S. Pat. Nos. 4,575,545 (Nakos et al.) and 4,575,546 (Klemarczyk et al.) attempt to extend the molecular weight range of UV curable silicone polymers by preparing block polymers consisting of acrylate-rich and acrylate-poor regions. However, these materials are difficult to prepare, and the relatively highly crosslinked acrylate-rich segments may be detrimental to the elastomeric properties of the cured silicone rubber.
It is therefore an object of this invention to provide silicone compositions which, even at high molecular weight, may be rapidly, completely, and reliably radiation cured.
It is also an object of this invention to provide radiation-cured silicone elastomers having properties which are equal to or better than those of prior art radiation-cured silicone elastomers.
It is another object of this invention to provide radiation-cured silicone PSAs having stable properties.
It is yet another object of this invention to provide silicone PSAs having improved tack properties and silicone elastomers having controlled mechanical properties.
It has been discovered that these and other objects and advantages which will become apparent from the following discussion may be achieved via the use of polysiloxanes having terminal groups which, in addition to being reactive, are also capable of intermolecular hydrogen bonding.